The present invention relates to a braking system for braking an electric DC motor.
Industrial machines that use moving parts are often provided with DC motors to impart movement to those parts. The DC motors are well suited for these applications because their torque and speed of rotation can be easily controlled. In turn, this enables to precisely control the rate of movement and the positioning of the moving part.
Once a moving part has been set in motion by a DC motor, that is usually effected through any suitable transmission system, the part must at some point be immobilised. This is usually the case of actuators designed to pick-up a component and deposit the component in a precise location. In fact, in many applications the ability to terminate the movement of the part repeatably is an important design consideration that determines the overall performance of the machine.
Several possible approaches to terminate the movement of a part or component driven by a DC motor exist. One approach relies on mechanical braking systems that dissipate the kinetic energy through friction between two contact surfaces. This strategy enables to very quickly decelerate the moving part, however, the braking system that can accomplish this task is complex and often not practical. Another approach is to control the DC motor so the latter decelerates the moving part through dynamic braking. In essence, when the movement of the part is to be terminated and power to the motor armature has been removed, a load is placed across the terminals of the DC motor. The former then ceases to act as a driving source and becomes a generator driven by the inertia of the part in motion. The electric current generated by the motor is dissipated in the load in the form of heat. This approach is widely used because the control mechanism for switching the motor from the driving mode to the braking mode is easy to implement. Indeed, it suffices to provide a suitable mechanism that will switch across the terminals of the motor, when braking is desired, a load to dissipate the energy.
To obtain a high rate of energy dissipation that is necessary to quickly stop the motor, the load across the terminals of the motor should be as high as possible. A high load (low impedance) is likely to generate high current though the winding of the DC motor that in turn can damage the permanent magnets of the motor because such high current can exceed demagnetisation levels. In practice, manufacturers of DC motors specify a peak current value that the motor can sustain without causing any damage to the magnets of the motor. Any braking system using this motor is thus designed not to exceed the peak current value, otherwise the motor can be damaged.
When a DC motor is operating in the dynamic braking mode, the current generated by the motor that is passing through the load reaches its peak value as soon as the braking mode is entered because at this point the rotary speed of the shaft is highest. As soon as the braking takes effect, however, the current progressively diminishes. The same relationship holds true with the rate of energy dissipation. The bulk of the energy is dissipated at the front end of the dynamic braking cycle, while less energy is dissipated near the end of the cycle. This observation highlights a fundamental deficiency in existing dynamic braking systems, where the average rate of energy dissipation during the braking cycle is relatively low, and cannot be increased to avoid exceeding the peak current limitation that occurs only during a small fraction of the cycle.
Under a first broad aspect, the present invention provides a braking system for a DC motor that is characterized by a maximum braking current value and that features first and second terminals. The braking system comprises a power supply that is capable of drawing electrical energy from the two terminals when the braking system is connected across these same terminals. A current control element receives electrical power for operation from the power supply. This current control element is capable of regulating the magnitude of a current passing through the windings of the DC motor when the braking system is connected across the first and second terminals, such that an average current passing through the windings of the DC motor during a braking cycle of the DC motor tracks the maximum braking current value over a major portion of the braking cycle.
The present inventor has made the unexpected discovery that the repeatability of movement of a DC motor can be significantly improved by dynamically braking the motor aggressively in order to reduce the unpowered coasting stroke. The unpowered coasting stroke has been found to be a source of repeatability error because of the duration of the coasting motion. During a long coasting motion, the impact of uncontrollable system variances such as friction shaft/bearings, among others, reduces repeatability. By reducing the time the motor spends in the coasting mode, the impact of those system variances is reduced. Consequently, the repeatability of movement is improved.
In a preferred embodiment, the braking system in accordance with the invention includes a current control element connected between the terminals of the DC motor to control the magnitude of the current passing in the motor windings during the braking mode of operation of the motor. The control strategy that the current control element implements is such as to allow the rate of energy dissipation during the dynamic braking cycle to be significantly increased by comparison to prior art devices, while preventing the system from exceeding the peak current value established for the motor.
In a most preferred embodiment, the system includes a current control element that can selectively acquire different operative states, namely a first operative state and a second operative state, in the first operative state the current control element manifesting a substantially lower impedance to the passage of current through the windings of the DC motor than in the second operative state. In other words, the operative states correspond to different levels of conduction; the current control element when in the second operative state allowing less current (or no current at all) to pass through the windings than in the first operative state. In a specific example, the current control element includes a semiconductor switch connected across the terminals of the DC motor. The semiconductor switch can be an N-channel MOSFET transistor which can acquire either one of the open, non-conducting condition (no current passing through it) and the closed, conducting condition (acting as a short circuit). The transistor can switch between the closed condition and the open condition, in the closed condition current being allowed to pass through it and also through the windings of the DC motor that act as energy dissipation elements by virtue of their inherent resistance.
In order to initiate the braking event, the DC motor is switched from the running mode, during which power is supplied to the motor windings, to the braking mode, during which the current control element is placed in series with the motor windings. The power supply module of the current control element quickly absorbs, filters and regulates some of the power generated by the motor, for supplying the braking circuit with sufficient power to energize the transistor with pulse width modulation (PWM) based oscillations. These oscillations remain in effect until almost the very end of the braking cycle. More specifically, throughout the braking cycle, the transistor will pulse on and off, according to a variable duty cycle. As the motor is braked, it is this duty cycle that will vary accordingly such that the average current flowing through the motor windings is maintained for as long as possible at a maximum allowable braking current value.
A control signal is provided to actuate the transistor. This control signal can be obtained from any suitable electronic device having the capability to react to a signal indicative of the current magnitude passing in the motor windings. Such signal can be obtained by placing in series with the transistor a resistor that generates an output voltage across its terminals proportional to the magnitude of the current passing through it. In a very specific example, the electronic device can be a comparator that observes the voltage drop across the resistor and compares this voltage drop to a set-point indicative of the maximum current that can be tolerated by the DC motor. The transistor acquires the open or the closed condition on the basis of a voltage signal received from the comparator. When the impressed voltage drop exceeds the set-point, indicating that the braking current flowing through the motor windings is above the safe level, the comparator causes the transistor to pulse with a duty cycle that is inversely proportional to the magnitude of the sensed braking current that is in excess of the set-point. Specifically, the smaller the magnitude of the sensed braking current that is in excess of the set-point, the larger the duty cycle. A larger duty cycle corresponds to more on time (when in the closed, conducting condition) and less off time (when in the open, non-conducting condition) for the transistor.
Thus, as the braking cycle progresses and the motor shaft speed is reduced, the on and off times of the duty cycle of the braking circuit adapt automatically to maintain an average maximum braking current through the motor windings. When the impressed voltage drop is below the set-point, indicating that the braking current flowing through the motor windings is below the safe level, the duty cycle of the transistor will become 100% (the transistor will remain on continuously with no pulsing effects) until the motor shaft speed slows to a complete stop.
In the example described above, the transistor is digitally controlled, in other words the transistor is maintained at the two extremes of its conduction range, namely the open condition or the closed condition. It is also possible to utilise an analog control scheme where the control signal causes the transistor to gradually change its level of conduction, so the current control element progressively swings back and forth between different levels of conduction, each level corresponding to a different operative state.
The current flow regulation effected by the transistor is related to the speed of rotation of the motor. When the motor speed is highest, which occurs immediately at the beginning of the dynamic braking cycle, the control effect exercised by the transistor is such as to repeatedly switch the transistor on and off according to a duty cycle that is characterized by a small on time and a large off time, such that the average current flowing through the motor windings tracks a constant maximum braking current value. This state of operation is maintained, although the duty cycle increases (more on time, less off time) as the motor speed decreases, while the speed of the motor is sufficient to generate voltage that would induce current in the electrical path of the motor windings above the safe level. However, once the speed of rotation of the motor reaches the equilibrium point, where the generated voltage would induce a current either equal to the maximal safe level or slightly below this level, the transistor becomes fully on without any pulsing effects (100% duty cycle) until the motor stops.
The braking system in accordance with the present invention is capable to terminate the rotary movement of the DC motor very rapidly while, at the same time, preventing damage to the motor. This is particularly useful because the range of applications of DC motors may now be expanded significantly and such motors can be considered to replace much more expensive servo or stepper motors. Such servo or stepper motors are normally reserved for applications where the ability to control the angular movement of the rotary shaft is critical. Those special motors are preferred over the more traditional DC motors because they have the ability to stop very quickly and generally speaking are very precise and offer excellent motion repeatability.
A DC motor provided with the braking control system in accordance with the invention now manifests a much better movement repeatability characteristics than a motor using no braking system or using a braking system constructed in accordance to prior art techniques. Repeatability of movement is an important parameter because in some applications repeatability is an acceptable alternative to precision. Repeatability means that the overall movement of the motor including the stroke under powered control and the unpowered coasting stroke are very consistent from one cycle to another. Although measurable and often substantial variations may exist with nominal stroke values, the actual movement is highly repeatable. In some industrial applications, servo or stepper type motors that are normally designed for precision of movement are the only choice not because they are precise but because they have good repeatability of movement characteristics as well. The braking system in accordance with the invention now allows DC motors to be considered for those specific applications. The obvious advantage to this novel approach is the significant cost savings resulting from the elimination of the servo or stepper type motors.
In a somewhat different embodiment, the current control element comprises a variable load whose impedance is modulated to progressively diminish as the speed of rotation of the motor is reduced. This embodiment uses a current measuring device that generates an output signal indicative of the magnitude of the current passing through the DC motor windings. The output of the current measuring device is supplied to a control module that includes two functional elements, namely a logic element and a variable load element. The logic element is preferably microprocessor based and includes an input to receive the output of the current measuring device. On the basis of the data supplied at the input, the microprocessor determines the value of the load and adjusts the load accordingly. As to the variable load element, this component may be formed of an array of resistors that can be selectively switched in parallel or series to adjust the impedance across the terminals of the DC motor. The microprocessor determines the number of resistors necessary to achieve the desired load value at any given time and issues an output signal to effectively switch on line the selected arrangement of resistors.